1. Field of the Invention
This invention relates to an optical system having a reflecting surface, and particularly to an optical system for forming the image of an object on the surface of an image pickup element such as silver halide film or a CCD by the use of an optical element provided with an incidence surface, an emergence surface and a plurality of reflecting surfaces on the surface of a transparent member. This optical system is suitable for a video camera, a still video camera and an image pickup apparatus such as a copying apparatus.
2. Related Background Art
There have heretofore been proposed various phototaking optical systems utilizing the reflecting surface of concave mirror, a convex mirror or the like. FIG. 9 of the accompanying drawings is a schematic view of a so-called mirror optical system comprising a concave mirror and a convex mirror.
In the mirror optical system of FIG. 9, an object light beam 104 from an object is reflected by the concave mirror 101, whereby it travels toward the object side while being converged, and impinges on the convex mirror 102. The light beam 102 is further reflected by the convex mirror 102, and hereafter it is imaged on an image plane 103.
This mirror optical system has the same construction as that of a so-called Cassegrainian type reflection telescope, and the optical path of a telephoto lens system comprised of a refracting lens which is long in the full length of the optical system is folded by the use of a reflecting mirror, whereby the full length of the optical system can be shortened.
Besides the Cassegrainian type, there are known a number of mirror optical systems of which the full length is shortened by the use of a plurality of reflecting mirrors.
Generally, in a mirror optical system such as the Cassegrainian type reflection telescope, there is the problem that part of the object light beam 104 is eclipsed by the convex mirror 102. This problem is attributable to the fact that the convex mirror 102 is in the passage area of the object light beam 104.
In order to solve this problem, there has also been proposed a mirror phototaking optical system used with the principal ray of the object light beam 104 spaced apart from an optical axis 105.
FIG. 10 of the accompanying drawings is a schematic view of a mirror optical system disclosed in U.S. Pat. No. 3,674,334, and the center axis itself of the reflecting surface of a reflecting mirror is made eccentric relative to an optical axis 114 and at the same time, the principal ray 116 of an object light beam 115 is spaced apart from the optical axis 114 to thereby solve the above-mentioned problem of eclipse.
The mirror optical system of FIG. 10 has a concave mirror 111, a convex mirror 113 and a concave mirror 112 in the order of passage of the light beam, but as indicated by dots-and-dash line, they originally are portions of reflecting mirrors rotation-symmetrical with respect to the optical axis 114. Of these, only the upper portion of the concave mirror 111 relative to the optical axis 114 as viewed in FIG. 10, only the lower portion of the convex mirror 113 relative to the optical axis 114 as viewed in FIG. 10 and only the lower portion of the concave mirror 112 relative to the optical axis 114 as viewed in FIG. 10 are used, whereby there is constructed an optical system in which the principal ray 116 of the object light beam 115 is spaced apart from the optical axis 114 and the eclipse of the object light beam 115 is eliminated.
FIG. 11 of the accompanying drawings is a schematic view of a mirror optical system disclosed in U.S. Pat. No. 5,063,586.
In FIG. 11, when an axis passing through the center of an object surface 121 perpendicular to the object surface 121 is defined as an optical axis 127, the central coordinates and center axes of the reflecting surfaces of a convex mirror 122, a concave mirror 123, a convex mirror 124 and a concave mirror 125 arranged in the order of passage of a light beam (the axes linking the centers of the reflecting surfaces and the centers of curvature of the surfaces) 122a, 123a, 124a and 125a are eccentric relative to the optical axis 127. In FIG. 11, the amount of eccentricity at this time and the radius of curvature of each surface are appropriately set to thereby prevent the eclipse of an object light beam 128 by each reflecting mirror and image the object light beam 128 efficiently on an imaging plane 126.
U.S. Pat. No. 4,737,021 and U.S. Pat. No. 4,265,510 also disclose a construction in which as in the optical system of FIG. 10, eclipse is avoided by the use of a portion of a reflecting mirror rotation-symmetrical with respect to the optical axis, or a construction in which as in the optical system of FIG. 11, the center axis itself of a reflecting mirror is made eccentric relative to the optical axis to thereby avoid eclipse.
FIG. 12 of the accompanying drawings shows an a focal optical system for observation using four reflecting surfaces disclosed in U.S. Pat. No. 5,309,276. In FIG. 12, four mirrors 201 to 204 are disposed so that a light beam from an object, not shown, lying at the left as viewed in FIG. 12 may be reflected by the first mirror 201, the second mirror 202, the third mirror 203 and the fourth mirror 204 in the named order, and may pass the front of the first mirror 201 twice, and then may emerge from the fourth mirror in a direction perpendicular to the direction of incidence of incident light 200 and may be imaged on an observer's pupil 205.
These reflecting optical systems have required many constituent parts, and to obtain necessary optical performance, it has been necessary to assemble respective optical parts with good accuracy. Particularly, the accuracy of the relative position of the plurality of reflecting mirrors has been severe and therefore, the adjustment of the position and angle of each reflecting mirror has been requisite.
As a method for solving this problem, it has been proposed to construct, for example, a mirror system having a plurality of reflecting mirrors (surfaces) by a single element.
As an element having a plurality of reflecting surfaces, there is a pentagonal roof prism used, for example, in a finder system or the like, or an optical prism such as a porroprism.
In these prisms, the plurality of reflecting surfaces are formed integrally with one another and therefore, the relative positional relation among the reflecting surfaces is made accurate and the mutual positional adjustment of the reflecting surfaces becomes unnecessary. The main function of these prisms is to change the direction of travel of rays of light to thereby effect the reversal of an image, and each reflecting mirror is comprised of a flat surface.
In contrast, there is also known an optical system in which a reflecting surface (reflecting mirror) formed on a prism is endowed with a curvature (refracting power).
FIG. 13 is a schematic view of the essential portions of an observation optical system disclosed in U.S. Pat. No. 4,775,217. This observation optical system for observing therethrough a scene of the outside and a display image displayed on an information display member as they are made to overlap each other.
In this observation optical system, a light beam 145 emerging from the display image on the information display member 141 enters from the incidence surface 148 of a prism, is reflected by a surface 142 and travels toward the scene at the left as viewed in FIG. 13, and enters a concave surface 143 comprising a half mirror. The display light beam 145 is reflected by this concave half mirror 143, and becomes a parallel light beam in which rays of light are substantially parallel to one another by the refracting power of the concave surface 143, and is refracted by and transmitted through the surface 142 to thereby form the enlarged virtual image of the display image, and enters an observer's pupil 144, and the display image enlarged thereby is recognized by the observer.
On the other hand, an object light beam 146 from the scene enters a surface 147 substantially parallel to the reflecting surface 142, and is refracted by the surface 147 and arrives at the concave surface 143 comprising the half mirror. The concave half mirror 143 has semi-transmitting film deposited by evaporation thereon, and a part of the object light beam 146 is transmitted through the concave half mirror 143, and is refracted by and transmitted through the surface 142 and enters the observer's pupil 144. Thereby, the observer visually confirms the display image in the scene of the outside as it overlaps the latter.
FIG. 14 of the accompanying drawings is a schematic view of the essential portions of an observation optical system disclosed in Japanese Laid-Open Patent Application No. 2-297516. Again through this observation optical system, the observer observes a scene of the outside and also observes a display image displayed on an information display member as it overlaps the former.
In this observation optical system, a display light beam 154 emerging from the information display member 150 is transmitted through a flat surface 157 constituting a prism Pa, enters the prism Pa and impinges on a reflecting surface 151 comprising a parabolic surface. The display light beam 154 is reflected by this reflecting surface 151 and becomes a convergent light beam, and is imaged on a focal plane 156. At this time, the display light beam 154 reflected by the reflecting surface 151 arrives at the focal plane 156 while being totally reflected between two parallel flat surfaces 157 and 158 constituting the prism Pa, whereby the thinning of the entire optical system is achieved.
Next, a display light beam 154' emerging as divergent light from the focal plane 156 impinges on a half mirror 152 comprising a parabolic surface while being totally reflected between the flat surfaces 157 and 158, and is reflected by this half mirror 152 and at the same time, forms the enlarged virtual image of the display image by the refracting power thereof and also becomes a substantially parallel light beam, and is transmitted through the surface 157 and enters an observer's pupil 153, whereby the display image is recognized by the observer.
On the other hand, an object light beam 155 from the outside is transmitted through a surface 158b constituting a prism Pb, is transmitted through the half mirror 152 comprising a parabolic surface, is transmitted through the surface 157 and enters the observer's pupil 153. The observer visually confirms the display image in the scene of the outside as it overlaps the latter.
Further, as an example using a prism-shaped optical element having a reflecting surface, there are optical heads for light pickup disclosed, for example, in Japanese Laid-Open Patent Application No. 5-12704 and Japanese Laid-Open Patent Application No. 6-139612. In these, light from a semiconductor laser is reflected by a Fresnel surface or a hologram surface, whereafter it is imaged on the surface of a disc, and the light from the disc is directed to a detector.
The present applicant has proposed an image pickup apparatus provided with an optical system having one or more optical elements having a plurality of curved or flat reflecting surfaces formed integrally with one another.
FIG. 15 of the accompanying drawings shows an example of this optical system and in FIG. 15, the reference numeral 51 designates an optical element in which a plurality of reflecting surfaces having curvatures are formed integrally with one another, and the element 51 is a transparent optical element having fine reflecting surfaces, i.e., a concave refracting surface R2, a concave mirror R3, a convex mirror R4, a concave mirror R5, a convex mirror R6 and a concave mirror R7, and a convex refracting surface R8 in succession from the object side, and the direction of a reference axis (light beam) entering the optical element 51 and the direction of a reference axis (light beam) emerging from the optical element 51 are substantially parallel to each other and opposite to each other. The reference numeral 52 denotes an optical correcting plate such as a rock crystal low-pass filter or an infrared cut filter, the reference numeral 53 designates the light receiving surface of an image pickup element such as a CCD, the reference numeral 54 denotes a stop disposed at the object side of the optical element 51, and the reference numeral 55 designates the reference axis of such a phototaking optical system.
In FIG. 15, light 56 from an object which is an object to be photographed has its amount of incidence regulated by the stop 54, and thereafter enters the concave refracting surface R2 of the optical element 51.
The light having entered the concave refracting surface R2 is made into divergent light by the power of the concave refracting surface R2, whereafter it is reflected by the concave mirror R3, is focused on an intermediate imaging plane N1 by the power of the concave mirror R3 and forms the image of the object on the plane N1.
The object light 56 temporarily imaged on the intermediate imaging plane N1 is repeatedly reflected by the convex mirror R4, the concave mirror R5, the convex mirror R6 and the concave mirror R7 and arrives at the convex refracting surface R8 while being affected by the power of the respective reflecting mirrors, and is refracted by the power of the convex refracting surface R8 and forms the image of the object on the light receiving surface 3 of the image pickup element.